1. Field of the Invention
The present invention relates to an automatic gain control (AGC) system and in particular to an AGC system using multi-chain signal detection and gain control.
2. Description of the Related Art
WLAN Receiver With Single Antenna: Overview
Receivers are ubiquitous circuits in wireless systems (e.g. such as wireless local area networks (WLANs)). FIG. 1 illustrates a simplified receiver 100 for receiving signals in a WLAN environment. In receiver 100, a bandpass filter 102 receives the incoming signals from an antenna 101 and outputs a predetermined band of frequencies (while excluding those frequencies higher and lower than the predetermined band). A variable gain RF amplifier 103 can provide an initial amplification to that predetermined band of frequencies. A mixer 104 converts those amplified signals (using a signal from a local oscillator) into intermediate frequency (IF) signals, which are then amplified by an IF amplifier 105.
In one embodiment, mixers 106 can include an in-phase mixer directly driven by a local oscillator and a quadrature mixer driven by the same local oscillator signal after it is phase-shifted by 90° in a phase shifter. In this way, in-phase (I) and quadrature (Q) components of the amplified IF signal are obtained at the outputs (only one shown) of mixers 106.
At this point, low-pass filters 107 (including both I and Q branches) can generate signals in the desired channel (called the baseband signals). Amplifiers 108 then amplify these baseband signals. Analog to digital converters (ADCs) 110 (provided for both the I and Q branches of low-pass filters 107) transform the amplified baseband signals into digital signals that can be analyzed by a processing block 111. ADCs 110 can be implemented as pipeline ADCs, sigma-delta converters, or any other mechanisms for converting analog signals to digital signals.
Of importance in receiver 100, less amplifier gain is needed for strong signals. Specifically, an amplified strong signal can distort incoming signals, thereby overloading and possibly damaging certain components (e.g. front-end components, such as the ADCs). For this reason, gain control 112 tries to maintain the amplified signals within certain ranges. Therefore, a gain control circuit 112 can be used to detect the magnitude of the digital signal, as measured by ADCs 110, and uses the detected magnitude to adjust the gains of RF amplifier 103, IF amplifier 105, and BB amplifiers 108. Notably, gain control 112 typically uses the same gain adjustment value for RF amplifier 103, IF amplifier 105, and BB amplifiers 108. To enhance the quality and performance of the wireless connection, it would be preferable to independently adjust the amplifiers in the receiver.
Moreover, some WLAN systems use multiple antennas to achieve diversity. Diversity can mitigate the effects of multipath. In multipath, one portion of a signal can travel directly to its destination and another portion of the signal bounces off an obstruction and then reaches the destination. Thus, in effect, portions of a signal take different paths. Portions of the signal taking the longer path can undesirably lose energy. Moreover, combining the received portions of the signal can result in some distortion, perhaps even causing bit errors in the signal. By using diversity, wherein the antenna having the strongest signal is selected, the WLAN system can increase the odds of minimizing multipath. Therefore, in a WLAN system having multiple antennas, gain adjustments are performed on the amplifiers based on information from the antenna receiving the strongest signal.
These “one size fits all” techniques fail to optimize amplifier performance. Therefore, a need arises for a WLAN receiver that can independently adjust all amplifiers, even those from multiple antennas, thereby optimizing the wireless connection.